Recently there has been a growing interest in 2,5-furan-dicarboxylic acid (“FDCA”) as an alternative monomer for the preparation of polyesters, polyamides, plasticizers and the likes. FDCA is obtainable from hydroxymethyl furfural, which can be produced from carbohydrates. In this way FDCA forms a biobased renewable alternative to other diacids, such a terephthalic acid, for the preparation of condensation polymers, such as polyethylene terephthalate.
The oxidation of hydroxymethyl furfural (“HMF”) is known from WO 2010/132740. This document discloses the batch-wise oxidation of HMF in the presence of an oxidation catalyst comprising cobalt, manganese and bromide. Products of such oxidation include FDCA. The document further teaches that when an alkoxymethyl furfural is used as feedstock, the product is predominantly the mono-ester of FDCA. This is exemplified by the oxidation of butoxymethyl furfural, which yields 5-(butoxycarbonyl)furan-2-carboxylic acid as the main product.
In WO 2011/043660 a batch process is described wherein a 5-alkoxymethylfurfural or a 2,5-(dialkoxymethyl)furan is oxidized with an oxidizing gas in the presence of an oxidation catalyst. The catalyst comprises cobalt, manganese and bromide. In the examples methoxymethyl furfural and ethoxymethyl furfural are used as feedstock. The reaction product in the examples is a mixture of FDCA and the mono-ester of FDCA, wherein FDCA is the predominant product.
WO 2012/161967 discloses an oxidation process of a starting material that may be HMF, an ether of HMF or an ester of HMF with an oxidizing gas and a catalyst system comprising cobalt, manganese and bromine at a temperature of 100 to 220° C. The process may be conducted in a continuous mode, e.g. in a bubble column. The experiments in this document confirm the results of the reaction in WO 2011/043660, in that the oxidation of ethoxymethyl furfural in a semi-batch process yields a mixture of FDCA and the mono-ethyl ester of FDCA, wherein the predominant product is FDCA. The mono-ethyl ester of FDCA is considered an undesired by-product. Another contaminant that was found is 5-formyl-furan-2-carboxylic acid. The document further discloses in an embodiment that the product of the oxidation may be separated into a low impurity slurry stream which is subjected to a secondary oxidation. It is observed that care should be taken about the amount of oxygen fed to the secondary oxidation since there exists a risk of burning the organic molecules to CO2.
The three prior art documents, i.e., WO 2010/132740, WO 2011/043660 and WO 2012/161967, all disclose that the oxidation reactions take place in a solvent. The most commonly used solvent is acetic acid or glacial acetic acid.
The experiments have been conducted in batch mode or semi-batch mode. None of the prior art documents refer to volatile impurities. It is believed that any volatile organic compound has been oxidized to CO2, as indicated in WO 2012/161967. Therefore, no problem seems to occur regarding volatile by-products.